Shock-wave study of the metallization of alkali halides up to 400 GPa

Alkali halide materials under high pressure are of fundamental interest, due to the number of phase transformations they exhibit under compression. For example, the phase transition from wide band gap insulator into electrical conductor, observed in many insulators under shock and static compression, has been poorly explored for many alkali halides. Results of early shock experiments pose a number of unresolved questions such as the possibility of non-equilibrium behavior at high shock pressures. In this study we investigate the optical properties of alkali halides NaCl, KBr, CsBr and CsI, under shock loading up to 400 GPa, by measuring shock wave speed and reflectivity using line VISAR in decaying-shock experiments. Significant increases in the optical reflectivity with shock pressure in all four cases indicate conditions of metallization at high pressures. The results are analyzed with respect to previous shock and static measurements on the alkali halides.